ABSTRACT Autism spectrum disorder (ASD) is a complex developmental disorder characterized by repetitive behavior and delayed social interaction and communication. The CDC estimates that ASD affects approximately 1 in 68 children, with symptoms appearing as early as 12 months, and continuing throughout the lifespan. Evidence- based treatment can enhance cognitive and adaptive functioning in children with ASD, but associated services are expensive, and financing for these services is inconsistent. Until recently, private health insurers often excluded ASD services from benefit plans, requiring many families to pay out-of-pocket. Even though 39 states have passed autism insurance mandates, more than half of children with ASD in the United States do not have private insurance that covers needed care. Many states use Medicaid Home and Community Based Services (HCBS) waivers both to expand eligibility for Medicaid-reimbursed services and to provide services not covered by their Medicaid plans under the premise that, without these services, certain individuals are at high risk of institutionalization. Medicaid is the largest US payer of healthcare for children with ASD, serving as much as 45% of US children with ASD. In our previous NIMH-funded study, we identified 50 current or former Medicaid waivers in 29 states that explicitly include children with ASD. We also collected detailed information about these waivers, including waiver inclusions and restrictions, estimated service provision and institutional costs, and inclusion of core ASD-related services. Ideally, these waivers would increase the number of children receiving Medicaid-reimbursed outpatient care for ASD and reduce their use of inpatient care. While we have done significant work examining the effects of these waivers on reducing unmet need, little is known about the effects of these programs on healthcare service use and costs. In an effort to optimize the quality and cost of Medicaid waiver services for ASD, this R01 proposal, which is responsive to PA-13-216 ?Research on Autism Spectrum Disorders? and to the Interagency Autism Coordinating Committee?s strategic plan objective of examining the implementation and dissemination of interventions and evaluating service cost effectiveness, would use up to 12 years of data from the Medicaid Analytic eXtract (MAX) files, merged with Medicaid waiver program data collected in our earlier study, to address this knowledge gap. Specifically, we will examine the characteristics of Medicaid HCBS waivers for children with ASD that affect Medicaid enrollment, healthcare service use, and Medicaid-reimbursed expenditures for children with ASD. This study will combine the largest national data source on healthcare experiences of children with ASD with comprehensive primary data on state policies that drive healthcare use for almost half of these children. We combine these data with a sophisticated, longitudinal research design to provide rigorous estimates of the effects of these waivers. The results will have important implications for national and state-level autism policy, as we identify mechanisms associated with more effective and efficient health services for children with ASD.